Page 217 - Advanced Mine Ventilation

Respirable Dust Sampling and Measurement 197

The core component of the TEOM is a vibrating hollow tube called the tapered
element. It is fixed on one end but free to oscillate at the other. The resonant frequency
of the element (approximately 252 Hz) is monitored by an electronic system along
with the oscillation amplitude. A small filter (16 mm diameter) assembly is mounted
on its free end, which collects the dust particles in the sampled air. As the weight of
the filter assembly changes owing to dust deposit, its frequency changes according
to Eq. (12.1).

" #
1 1
DW ¼ A 2 2 (12.1)
f t f o

where DW is the change in the weight of the filter, mg; f o , initial frequency in Hz; f t ,
measured frequency in Hz after dust deposition in time, t; A, a constant that converts
change in frequency to weight gained.
The respirable dust concentration is calculated from the mass of dust collected, rate
of air flow, and time of sample collection. The personal dust monitor (PDM) collects
respirable dust through a HigginseDewell (HD) cyclone that follows the International
Organization of Standardization (ISO) convention for respirable dust. The ISO stan-
dard is slightly different from the AEC (Atomic Energy Commission) curve used in
the personal dust sampler [7]. PDM draws air at the rate of 2.2 L/min. The air intake

is heated to 46 C to keep moisture out and minimize the error due to excessive mois-
ture in mine air.
PDM readings are made unreliable by a variety of confounders. The main sources
of error are as follows:
1. Moisture in air and the coal dust sample.
2. Atmospheric temperature fluctuations.
3. Mine air humidity.
4. Air flow fluctuations.
5. Pressure drop across the filter element.
6. Heat-induced loss of volatile matter in coal.
PDM is powered by the miner’s lamp battery. An illuminated data display on the
PDM continuously shows the previous 30 min dust concentration, cumulative mass
concentration for the shift, and a projected end of 8-h shift concentration. A full
description of the PDM is available in the literature [8,9].
A summary of all dust measuring instruments is presented in Table 12.1 for a ready
reference.

12.4.1 Comparison of PDM with a Personal Dust Sampler and
Marple Impactor
An extensive study by the National Institute for Occupational Safety and Health
(NIOSH) [10] shows that the PDM does have a linear relationship with the standard
personal dust monitor and the Marple impactors (MIs) with some qualifications.